A Fabry-Perot (FP) filter is made of a pair of mirrors separated by a selected optical path length which form the resonance cavity of the filter. A FP filter passes a narrow band of light determined by the reflectivities of its mirrors which satisfy a resonance condition such that when the optical length of the round-trip length of the cavity is an integer of a wavelength, then that wavelength together with a narrow band resonates inside the cavity, and passes through the filter with no (theoretically) or very low loss.
For a fixed FP cavity length, the resonant wavelength changes periodically. The period of the resonant wavelength is called free spectral range (FSR) of the filter. FIG. 3A is a representative FP filter output which illustrates the definitions of FSR and bandwidth (BW) of such a filter. Finesse () is a function of BW and FSR and is expressed as: =FSR/BW. FIG. 3B illustrates the glitch free dynamic range (GFDR) of a filter. The GFDR characterizes the level of the undesired multi-mode FP structure relative to the fundamental mode transmission peak, and is defined as the ratio of the peak value of the resonance mode to the value of the peak spurious spectral content (measured over the entire FSR).
By changing cavity length, the FSR of an FP filter can be changed, and the resonant wavelength can be tuned. Three types of FP tunable filters are typically used in fiber-optic communications. One is a lensed FP interferometer, another is a microelectromechanical system based FP filter (MEMS-FP filter), and the other is an all fiber FP interferometer.
In a lensed FP filter, the light propagating inside a single-mode fiber is first collimated by a (Gradient Index) GRIN lens, then enters a bulk optic FP cavity, and the transmitted wavelength is coupled back to a single mode fiber by another GRIN lens. The expanded beam size of such a filter results in reduced stability and optical performance, and also limits the filter FSR that can be achieved. A MEMS-FP filter uses curved and suspended dielectric mirrors, and as a result tends to suffer from mechanical and thermal instabilities.
An all fiber Fabry-Perot tunable filter (FFP-TF), illustrated in FIG. 1, is an all-fiber device that consists of two mirrors (10 and 12) deposited directly onto fiber ends (9 and 11, respectively), a single-mode fiber (SMF) waveguide (20, 5 μm to 10 mm in length, also called a wafer) of selected length, bonded to one mirror (10. the embedded mirror). The internal end of the wafer (13) and the mirror-ended fiber end (11) are spaced apart to form a 1-2 μm air-gap (21) within the cavity. To provide tunability, a FFP may be designed such that the length of the air gap in the resonance cavity may be selectively adjusted to tune the filter. The FFP configuration of FIG. 1 is illustrated as a fiber ferrule assembly in which fibers (5 and 7) each having a fiber core (22) and fiber cladding (23) are fixed within the axial bores of ferrules (1 and 3). Wafer 20 is formed by aligning and bonding the ends of two ferrule confined fibers and cutting one to the desired wafer length to provide the wafer (20) bonded to the ferrule (1). The entire optical configuration is aligned within a fixture or holder which maintains fiber alignment and allows the cavity length to be tuned without significant loss of alignment. For example, the holder is provided with a piezoelectric actuator (PZT) to allow the cavity length to be changed and, thus, provide wavelength tuning and control with positioning accuracy of atomic dimensions.
FIG. 2 provides a schematic illustration of an alignment/tuning fixture 40 for FFP filters. Ferrules 1 and 3 (containing fibers 5 and 7, respectively) are held within ferrule holders 35 and 37 of fixture 40 with internal ends aligned and spaced apart to form the air gap. Ferrule holders 35 and 37, into which the ferrules are inserted and held in alignment, are attached on either side of a PZT element (36) which changes its length (along its axis 25) on application of a voltage. The PZT element (36) has an axial bore into which the ferrules extend and within which the FFP cavity is formed. Fixed frequency Fiber Fabry-Perot filters with a resonance cavity having a fixed optical path length have also been described.
Examples of fixed and tunable FFP filters and holders for alignment and tuning of such filters are provided in U.S. Pat. Nos. 5,212,745; 5,212,746; 5,289,552; 5,375,181; 5,422,970; 5,509,093; 5,563,973; 6,241,397; and U.S. patent application Ser. No. 10/233,011. Fixed frequency and tunable FFP filters have various applications in the fields of sensing and telecommunications as exemplified in U.S. patents including 5,892,582; 6,115,122; 6,327,036; 6,449,047; 6,137,812; 5,425,039; 5,838,437; and U.S. patent application Ser. Nos. 09/633,362; 09/505,083; 09/669,488. All of which are incorporated by reference in their entireties herein to the extent that they are not inconsistent with the present application.
FFP-TFs are robust and field-worthy compared to lensed and microelectromechanical Fabry-Perot interferometers. The key to the stable, high performance characteristics of the all-fiber filters is the presence of the internal SMF waveguide (wafer) that provides intra-cavity waveguiding, eliminates extraneous cavity modes, and eases mirror alignment required for high-finesse and low-loss operation. Total fiber length inside the wafer or waveguide within the FFP cavity ranges from a few micrometers to a few millimeters for telecommunication components. An important characteristic of FFP-TFs is the high accuracy with which measured response corresponds to theoretical response. However, due to the high precision requirements for making a wafer or waveguide of selected length, it becomes very difficult to fabricate wafered cavity filters with wide-FSR (>100 nm).
To maximally utilize the channeling capacity that single mode optical fibers can offer, the telecom industry is exploring systems that utilize wavelengths at both C (central) and L (long) bands (1520-1620 nm), and will soon expand these systems to the S (short) band (1420-1520 nm). One application for FFP tunable filters is as spectrum analyzers for channel and performance monitoring in a network. In order to monitor both the C and L bands, the FSR of the filter should be greater than about 115 nm. To achieve such a wide FSR in an FFP-TF configuration using a wafered cavity, a wafer length of less than about 3.5 micron is needed. In addition to the difficulties inherent in the reproducible manufacture of fiber wafers of short and precise length, the beam shaping effect of the wafer decreases with its length and as a result filters having such short wafers can exhibit high sensitivity to alignment offset. Decrease in manufacturing yield due to machining imperfections and difficulty with fiber alignment makes production of wafered FFP filters for applications requiring a FSR greater than about 100 nm impractical.
The present invention provides improved FFP filter design, particularly for high FSR applications, which overcome the problems encountered in FFP filters having watered cavities. In addition, the invention provides wafered FFPs with lower light losses and decreased sensitivity to alignment offset. These improved designs rely generally on shaping or contouring of at least one of the fiber ends that form the FFP cavity. More specifically, as described in more detail below, a mirrored fiber end of the FFP cavity is shaped so that the fiber core end and the fiber cladding end of the fiber end are shaped or contoured differently. Preferably the fiber core end is concave and the fiber cladding is flat, convex or concave with a curvature that is different from that of fiber core end.
It is known in the art to shape fiber ends in fiber couplers, for example, to decrease pressure at the end faces of the fibers and decrease light loss (U.S. Pat. Nos. 5,887,099; 5,796,894) by use of a planar undercut or to reduce back reflection (U.S. Pat. No. 5,037,176) by providing the fiber end with a concave polish. It is also known in the art to employ convex spherical shaping (doming) at a fiber end in a coupler (see Background section of U.S. Pat. No. 6,113,469). Various methods are known in the art for shaping the end of a fiber. Typically, the fiber is inserted into a fiber ferrule and the ferrule end and fiber end may shaped by polishing or grinding methods known in the art of fiber optics.
U.S. Pat. No. 6,445,838, issued Sep. 3, 2002, relates to fiber Fabry-Perot resonators in which a plurality of fiber retainers are disposed on a substrate for mounting and aligning the fiber Fabry-Perot (FFP) resonator. A pair of “binders” are “disposed on the substrate . . . proximate selected opposed pairs” of fiber retainers. FIG. 4 of the patent illustrates the use of a concave mirror-coated fiber end in the formation of the FFP resonator. A “flat-concave” configuration and a “concave-convex” configuration are reported. The “flat-concave” configuration is reported to be a “very efficient resonator.” FIG. 4 illustrates fibers having a concave end indicating the fiber core and the fiber cladding. It is noteworthy that no distinction is made in the patent between the shaping of the fiber core end and the fiber cladding end. FIG. 4 illustrates a continuous concave curve over the entire fiber end with no difference in curvature between the core end and the cladding end. In a configuration reported to be preferred, the curvature radius of the fiber end is made to “match the curvature of the wavefront” inside the cavity. The shaping of the fiber end is reported to limit the transmission loss of the resonator.